1. Field of the Invention
This invention relates generally to semiconductor fabrication, and more particularly to a method of removing residues from etched metallic surfaces.
2. Description of the Related Art
The fabrication of multi-level metallization structures in integrated circuit processing involves the alternating and sequential fabrication of various metal and interlevel dielectric layers. Interconnections between adjacent metal layers are ordinarily established by means of a plurality of contacts or local interconnects extending through the interlevel dielectric layers that separate the metal layers.
The fabrication of metal interconnect layers involves the sequential blanket deposition of metallic material, lithographic resist patterning, and etch to remove the unmasked portions of the deposited metallic layer. Following the etch, the resist is stripped and the remaining surfaces cleansed to remove any residual resist material remaining after the stripping process. Many conventional post metal etch stripping processes have traditionally used a plasma strip followed by a solvent cleaning process. The use of amine based solvent chemistry as a post metal etch cleaning agent has long been a standard solvent cleaning technique used in the semiconductor industry. Conventional processes have employed both hydroxyl and primary amines as cleansing solvents.
Solvent stripping processes provided adequate residue removal in earlier fabrication processes involving relatively large geometries. However, more modern processes involving shrinking line width and device designs incorporating misalignments between metal interconnect layers and underlying contact structures have exposed certain shortfalls associated with solvent based chemistry. One such shortcoming is the propensity for traditional solvent based chemistries to etch the metal films exposed by both the plasma strip and solvent stripping processes. The undesirable erosion of both the overlying metal interconnect structures as well as the exposed portions of contact structures, such as tungsten plugs, is the product of both purely chemical and electrochemical mechanisms. The chemical erosion of the metallic interconnect and contact structures is largely the result of the chemical interaction of the residual OH molecules generated by hydroxyl amine solvents, although primary amines can result in a chemical erosion as well. The free hydroxides cause the pH of the solvents to be relatively high. The electrochemical attack of underlying contact structures is due to an electrogalvanic reaction that occurs as a result of charges stored in conducting layers positioned beneath the contact structures during the plasma etching of the overlying metallic interconnect layer. The subsequent use of an amine based solvent triggers the electrogalvanic reaction that can significantly erode the exposed portions of the contact structures that connect the overlying interconnect layer to the underlying conducting layers.
The use of a water and carbon dioxide post solvent strip rinse following solvent cleaning has been employed in some conventional resist stripping processes to ameliorate the potentially damaging effects of amine based solvent chemistry. The goal of utilizing CO.sub.2 and water is to reduce the pH of the water during the water rinse process. Although this technique has shown some promise in alleviating some of the chemical and electrochemical processes that can erode the metal interconnect and underlying contact structures, the potential for such unattended erosion of those conducting structures has not been completely eliminated.
Aside from the unwanted chemical effects, the use of solvent chemistry as a cleansing agent involves manufacturing costs. The solvent rinse process consumes time and thus can constitute a significant limit on throughput. Furthermore, solvent consumption can be significant as solvent recycling is problematic.
One manufacturer of resist stripping tools has turned away from solvent based chemistry in favor of a solventless cleaning process. In this conventional process, one or more plasma strip processes are employed following metal etch and the last plasma strip process is followed by a water rinse. The goal of this alternate process is to rely on the effectiveness of the plasma strip and water rinse process to remove as much resist residue as possible without the incorporation of solvent based chemistry. The difficulty with this alternate process is that resist residues frequently remain on the metal interconnect structures following the plasma strip and water rinse process. The presence of resist residues on the remaining metal interconnect surfaces following plasma strip and water rinse is thought to be the result of unavoidable variations in the kinetics of the plasma strip process. The behavior of the particles within the plasma is influenced by a large number of different mechanisms, each of which is subject to variation, such as the electric and magnetic fields acting on the plasma, the pressure within the chamber, the temperature of both the plasma and the impacted surfaces, as well as the myriad of various chemical reactions that frequently take place within and around the plasma.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.